Factors Influencing the Functions of Optical Brighteners

Last Updated on 26/01/2021

Optical Brighteners:
Optical brighteners or whiteners are chemical compounds that absorb invisible ultraviolet radiation from daylight and re-emit it as visible blue fluorescent light. The optical brighteners counteract the yellowness of the fabric by increasing the reflection of blue light rays. They convert invisible short-wave ultraviolet rays of sunlight into visible blue light and has a degree of whiteness which is comparatively more intense.  OBA’s take up invisible UV light in the spectral band of 300–400 nm and, owing to the Stokes’s shift, re-emit it in the visible higher wavelength spectrum of approx. 400–450 nm.

Optical Brighteners
Fig: Optical brighteners

Factors Influencing the Functions of Optical Brighteners:

Optical brighteners or optical whiteners are applied to substrate as a separate after-treatment process or are incorporated into bleaching and finishing baths. Since the fluorescent brightening agents behave like dyestuffs, their efficiency and effectiveness are influenced by various factors that are important in application.

The brightening effect is dependent on the nature of the substrate. For example, a very strong reflectance is observed with whitened cotton, but it is weaker in viscose and wool. All synthetic fibres absorb strongly in the near ultraviolet region. Since the fluorescence produced by optical brightening agent is added to reflectance of the substrate, the maximum fluorescence effect is achieved on those substrates whose ability to absorb the ultraviolet region is suppressed by chemical brightening. In the absence of sufficient affinity of brighteners, the application results in yellow to green colour yield.

chemical structure of Optical Brighteners
Fig: Chemical structure of optical brighteners

There is a saturation limit for each optical whitening agent. Above certain concentration on the fibre a yellow colour is superimposed on the flourescence resulting in decrease in whiteness. This is because at higher concentration of brighteners a protective optical layer (filter) is formed on the surface of the substrate which prevents the extinction of the molecules of the brightening agent in deeper layers (so called self-quenching, concentration quenching of fluorescence or filter effect).

Method of application
The saturation limit of an optical brightening agent, however, is also dependent on the method of application to the substrate. Usually exhaust application process gives higher whiteness value then it does when apply by padding technique for a given amount of whitener.

Generally optical brightening agents have high rate of exhaustion on the substrate and therefore great care is to be taken to avoid unlevel application. Slow exhaustion rate and increased migration time is necessary to produce level whiteness on the fabric.

The optimum temperatures of optical brightening agents on cellulosic fibres are usually between 40 and 60~ and further rise in temperature tend to lower the exhaustion. However, for synthetic fibres higher temperature is needed for good penetration of the brighteners.

The chemical stability, solubility and affinity of optical brightening agents depend on effective pH value in solution. For example, for wool and polyamide fibres, optimum pH is on the acidic side for better exhaustion.

Generally salt is added in the application bath to promote and also to control the rate of exhaustion of the brighteners on cellulosic fibres.

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